The invention finds a particularly important though nonexclusive application in the non-real-time utilization of the images provided by an Earth observation instrument carried by a satellite or an aircraft moving along a substantially known trajectory.
The reconstruction of the variations in the orientation of the line of sight of the instrument in the course of successive image sequences is essential for the utilization of the images obtained, in order to restore the geometry thereof and be able to locate the objects observed in a georeference frame with the desired degree of accuracy.
Restoration of the orientation at each image sequence with respect to a reference frame is required in particular in order to achieve good performance in the case of satellites that use an instrument for taking pictures in the visible/IR region.
For each image sequence, the instrument will be oriented and stabilized in a given direction, possibly with a small residual rotation speed controlled by the user. During image sequences, the information delivered by the strip-detectors will be stored and then forwarded and processed on the ground in order to reconstruct two-dimensional images of the zones observed.
The reconstruction of these images requires restoration of the movements of the line of sight during image sequences, this line of sight not being absolutely stable during image sequences, on account of residual satellite movements in roll, pitch and yaw, in a frequency span that may reach as much as several hundred hertz on account of high-frequency disturbances (vibrations generated by certain equipment). Moreover, purely geometric effects have to be taken into account. For example, taking pictures of a band of terrain that exhibits a craggy relief that is not necessarily known leads to different effects of the variations in line of sight depending on the altitude of the point observed.
According to the prior art, the orientation and angular movements of the line of sight are determined at any instant during image sequences by using position and angular velocity sensors, such as star sensors and gyroscopes. However, the increasing resolution of instruments requires ever more accurate attitude restoration that is difficult to achieve with such sensors. In actual fact, star sensors provide a very high attitude measurement accuracy, but within a relatively low frequency range (typically below a few hertz). Gyroscopes are likewise limited in terms of bandwidth (typically less than a few tens of hertz), often because of their electronics. Moreover, neither of these two types of sensors provides a direct measurement of the line of sight of the instrument because, by their very nature, they only provide information about their own orientation. This is particularly limiting when dealing with the restoration of movements of the line of sight at high frequency, typically 50-150 hertz, since here the vibrations at these frequencies affect the line of sight of the instrument and the sensors themselves differently.